The present invention relates generally to the field of temperature measurement and thermocouple devices. More particularly, the invention relates to a novel technique for effectively determining thermocouple cold junction temperature in a multi-channel terminal configuration.
Thermocouples are devices used to measure temperature and are one of the more versatile temperature sensors available. These temperature sensors or transducers are generally rugged and relatively inexpensive, and may be constructed of various metals. Thermocouples may be used to measure a relatively wide range of temperatures (e.g., −200° C. to 2600° C.) in a variety of applications and environments. In general, thermocouples rely on the principle that a voltage potential occurs when there is a temperature gradient along the length of a conductor.
A thermocouple device is formed by joining two conductors or wires of dissimilar metals to form a junction of the two wires called a measuring junction (or sensing junction, and so on). Although almost any two types of metal can be used to make the thermocouple wires, a number of standard types are used because they possess predictable output voltages and can handle large temperature gradients. The several types of thermocouples available may be designated by capital letters that indicate their composition according to American National Standards Institute (ANSI) conventions. For example, a J-type thermocouple has one iron conductor and one constantan (copper-nickel alloy) conductor.
The thermocouple measuring junction may be encased in a sensor probe, for example, with the probe positioned at the point of temperature measurement (i.e., at the temperature source). In principle, as the temperature of the measuring junction changes with the temperature source, a temperature gradient is formed (along the wires) between the measuring junction and the opposite free ends of the two wires. Advantageously, a predictable thermoelectric voltage is generated as a function of this temperature gradient. By taking into account the composition of the two dissimilar metal wires, this generated thermoelectric voltage (sometimes called the “Seebeck” voltage) can be related to the temperature gradient along the wires. This temperature gradient is summed with a reference or “cold” junction temperature to give the temperature of the source being measured. Thermocouple measurements typically require sensing or determining the reference temperature (the cold junction temperature) where the thermocouple wires connect to the voltage measurement system.
In construction, the two free ends of the thermocouple wires may be connected at a voltage measuring instrument (e.g., an analog to digital instrument, voltmeter, control instrument, temperature control module etc.) to measure the thermoelectric voltage. The connection of these two wires to the voltage measuring instrument is accomplished by using an electrical connector. An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection formed using the electrical connector may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or devices. There are hundreds of types of electrical connectors, including such devices as terminal blocks, posts, crimp-on connectors, insulation displacement connectors, plug and socket connectors, blade connectors, and ring and spade terminals. Sometimes, the electrical connectors are divided into two portions, with each portion connected with one of the two wires or devices to be connected to each other. Sometimes, the two portions of the electrical connector form a male/female arrangement.
When the two free ends of the thermocouple wires are connected at a voltage measuring instrument (e.g., an analog to digital instrument, voltmeter, control instrument, temperature control module etc.) to measure the thermoelectric voltage, the two free ends of the thermocouple wires are connected to the voltage measuring instrument using a connector, such as a terminal block, and forms a second junction of the thermocouple wires called a reference junction (or cold junction). The connection of these two wires at the connector forms the second junction of the wires called the reference junction (or cold junction). The term “cold junction” comes from the traditional practice of holding this reference junction at zero degrees Celsius in an ice bath. However, maintaining an ice bath is not practical for most measurement applications. Thus, the actual temperature of the point of connection of the thermocouple wires to the measuring instrument is measured and recorded. As discussed above, the cold junction may be formed at the electrical connector which connects the two free ends of the thermocouple wires to the voltage measuring instrument. Sometimes, the electrical connector, and therefore the cold junction, is a single piece, such as with a terminal block, and other times, the electric connector and the cold junction may be divided into two or more pieces or portions, with each portion connected with one of the two wires or devices to be connected to each other.
Typically cold junction temperature may be sensed by a thermistor or other temperature sensor or device such as a semiconductor temperature sensor, which is in relatively good thermal contact with the input connectors of the measuring instrument. Again, this second temperature reading, the cold junction temperature, is used by the measuring instrument to calculate the true temperature at the thermocouple tip, the measuring junction.
In sum, to determine the measured temperature of the source (i.e., the component, process, system, equipment, etc.) at the measuring junction of the two wires, the calculated temperature gradient based on the measured voltage is summed with the cold junction temperature (e.g., measured with a thermistor) at the voltage measuring instrument or terminal block. Thus, in operation, a thermocouple measures temperature by generating a voltage (e.g., millivolt) proportional to the temperature difference between the measuring and cold junctions of two dissimilar metals. For smaller changes in temperature, the voltage is substantially linearly proportional to temperature difference. For relatively larger changes in temperature, the relationship may become non-linear.
In ever-increasing demanding applications in precision temperature measurement, and with equally-demanding desires to reduce costs, the cold junction temperature measurement can be problematic. In a variety of configurations, errors in the measurement of the cold junction temperature give errors in the measurement of the source temperature. A typically accurate but expensive technique is to use a high-accuracy thermistor affixed to each terminal or to the channel that contains the two terminals. A less expensive but also less accurate solution is a single integrated circuit sensor. A particular problem is with electrical connectors which have multiple portions. For example, an electrical connector may have a first portion connected to a circuit board encased within a housing, and a part of that first portion may even be extending from the circuit board and out of the housing, and a second portion which is separate from the first portion and which may connect to the first portion outside of the housing and away from the circuit board.
Relatively significant temperature gradients may exist across both portions of the electrical connector, including the first portion which is connected with the circuit board within the housing and between the components within the housing and the second portion which resides in the atmosphere outside the housing. There is a need, therefore, for cost-effective and accurate measurement of cold junction temperature in an electrical connector having multiple portions.